in a normal MCDU you have departure area transition altitude (from low to high, local QNH to STD QNH 1013.25 hPa/29.92 InHg) and in arrival area which might be in other place in the world you have a transition level from high to low, STD 1013.25/29.92 to local altimeter setting accordingly to local QNH.

It might be useful for FMC to store both of them

We really need datarefs for that purposes : transition altitude and transition level and boleans
one for altimeter shows real altitude(true/false) and one for current flight level mode (true/false).